One of the most common and important problems in the application of motors is the temperature rise resulting from various losses, such as copper losses, brush-contact loss, core loss, mechanical loss, stray load loss, etc. And, as well known in the art, the operating temperature of a motor is closely associated with its life expectancy, because deterioration of the insulation is a function of both time and temperature. Therefore, various cooling means or ventilating systems are proposed for such machines to remove the heat arising from the inevitable losses, and thus to prevent overheating of the machines.